Understanding Storage of Electricity, Quantities and Costs

The ability to store electricity is critical to any proposal that relies on wind and solar to supply a major proportion of the nation’s electricity.

Like so many details about renewables, most people don’t have the time to look behind the headlines.

When Pedro Pizarro president of Edison International said California would need 10 gigawatts of storage for California to cut greenhouse gas emissions 40% by 2030, few people had any understanding of what was needed to create 10 gigawatts of storage.

But the statement itself was unclear, because it was not known how long the stored electricity would be available.

Mr. Pizarro might have assumed, based on past practices with batteries, that the electricity would be available for one hour, in which case the correct notation would have been 10 GWh.

Since Mr. Pizarro made no mention of time, his statement was confusing.

Hoover Dam. Photo by D. Dears

Storage could be provided by several methods: It could be provided by pumped storage, compressed air storage, storage using batteries, or any of several other methods.

It should be noted that we are actually discussing the storage of energy. In the case of pumped storage, it’s the storage of kinetic energy. In the case of batteries, it’s typically the storage of chemical energy.

In each instance, the stored energy can be released as electricity.

For reference, but not essential for understanding storage, here are fundamental equations:

P=E x I x PowerFactor: Or, Power in watts = Voltage in volts x Current in amps x PF

MW = 1,000,000 watts

Energy = power x time

Before evaluating storage to see whether it is a viable option for augmenting wind and solar power on the grid, let’s examine some fundamentals.

Storage has several dimensions.

The first dimension is the size of storage, e.g., MW, while the second is the length of time the stored electricity is available, e.g, MWh.

The key measurement isn’t MegaWatts (MW) but MegaWattHours (MWh), i.e., the amount of time the stored electricity is available for use at a specific rate.

MWh represents the amount of energy stored by the battery.

Additional dimensions relate to cost and life.

Two other factors that need to be considered when analyzing storage are:

How much electricity can flow from storage during any time period, which is the rate of flow.

The efficiency of the system, which accounts for losses.

One of the largest storage batteries being installed in California is the 400 MWh project by Southern California Edison. It consists of a 100 MW battery that can supply electricity for 4 hours.

Another storage installation is rated 20 MW for 15 minutes, or 5 MWh.

Reducing storage to simpler terms:

A 1kWh (i.e, 1,000 Watt-hour), battery could either power one, 100-watt bulb for 10 hours, or ten, 100-watt bulbs for one hour.

Battery Storage for a Home

Battery storage has been proposed for homes, typically in conjunction with PV rooftop solar panels, but it could also be installed to supply electricity when there is a blackout.

Tesla’s Powerwall Lithium-ion battery would be an option.

But how long could one Powerwall battery supply electricity if there were a blackout?

Powerwall-2 is rated 13.5 kWh and can probably provide electricity to the home for about six hours during a blackout. (http://bit.ly/2jt53wv)

The cost for this single Powerwall-2 battery rated 13.5 kWh would be $7,200 including installation. Therefore, the cost of storage for this home would be $530 per kWh.

Cost of battery storage

Any discussion of storage should also cover its cost and the life of the battery.

Referring to the 10 gigawatt storage required for California to cut its GHG emissions 40% by 2030, and assuming the storage was for only one hour, or 10 GWh, the cost of storage at $300 per kWh would be $3,000,000,000.

(For reference, the Li-ion batteries for a proposed 1GWh installation in Korea are to cost $300 million. http://bit.ly/2zIfKB6 )

But the life of Li-ion batteries has been estimated to be 10 years.

If the life of battery storage is ten years, an additional $3 billion will have to be invested every ten years to maintain the same level of storage.

Without making any judgments about whether storage is worthwhile, the basic elements of storage using batteries are:

The amount of electricity available from a battery

The length of time the electricity is available

The cost of storage

The life of the storage installation

Every proposal for storage can be evaluated based on these four criteria, while also keeping in mind any limitations such as efficiency.

Media articles about storage must address all four criteria if the report is to be credible.

Referring to 10 GW of storage, as mentioned earlier, leaves too much unknown. Is the size of the storage facility only 1 GW, making this amount of electricity available for 10 hours, or is it 10 GW available for only one hour?

Without all the information, it’s impossible to know how realistic any storage proposal is … or what its cost will be over ensuing decades.

Too many articles make it seem as though it would be a simple matter to establish enough storage to allow wind and solar to provide all our electricity, or even most of it.

But this is a fantasy that is quickly apparent when all the facts are known about the amount of storage needed, in terms of MWh, and how much it will cost.